Semiconductor components, such as packages, dice and wafers can include external contacts in the form of solder contact balls. The contact balls are in electrical communication with integrated circuits, and other electrical elements, contained on the components. For some components, such as chip scale packages and BGA packages, the contact balls can be arranged in a dense grid array, such as a ball grid array (BGA), or a fine ball grid array (FBGA). The contact balls provide a high input/output capability for a component, and permit the component to be surface mounted to a supporting substrate, such as a printed circuit board (PCB).
FIG. 1A illustrates a contact ball 10A bonded to a bonding pad 12A on a semiconductor component 14A. In this example, the component 14A comprises a semiconductor package, such as a chip scale package, or a BGA package. The bonding pad 12A comprises a BGA pad formed on a backside of the component 14A out of a solderable metal such as molybdenum or copper.
One conventional method for attaching the contact ball 10A to the component 14A uses a solder reflow bonding process. With this method the contact ball 10A is formed separately out of a non-eutectic solder alloy such as 95% Pb/5% Sn, 60% Pb/40% Sn, 63% Sn/37% Pb, or 62% Pb/36% Sn/2% Ag. Typically, the contact ball 10A has the shape of a sphere, or a truncated sphere.
Initially, a layer of eutectic solder can be deposited on the bonding pad 12A using a suitable deposition process such as screen printing to form a eutectic solder fillet 16A. Typically, the eutectic solder is in the form of a paste. A platen can be used to hold the component 14A, while the eutectic solder is deposited on the bonding pad 12A.
Alternately, a flux (not shown) can be applied to the bonding pad 12A. The flux chemically attacks surface oxides, such that the molten solder can wet the surfaces to be bonded. The flux also performs a tacking function prior to solder reflow. Following application of the flux, the contact ball 10A can be placed on the bonding pad 12A in physical contact with the eutectic solder and flux. A fixture can be used to center and maintain the contact ball 10A on the eutectic solder paste and bonding pad 12A.
Following placement of the contact ball 10A on the bonding pad 12A, the component 14A can be placed in a furnace at a temperature sufficient to reflow the eutectic solder and form the fillet 16A. The eutectic solder fillet 16A metallurgically bonds the contact ball 10A to the bonding pad 12A. The component 14A can then be removed from the furnace and cooled. In addition, the excess flux can be removed from the exposed surfaces of the component 14A and the contact ball 10A, using a suitable cleaning agent.
Suitable furnaces for performing the reflow process include convection ovens and infrared ovens. Rather than an oven, the bonding process can be performed using a pulse-thermode, a hot-air thermode, or a laser. A solder ball bumper, for example, uses a laser to form the eutectic solder fillet 16A, and bond the contact ball 10A to the bonding pad 12A. Alternately, the contact ball 10A can be bonded to the bonding pad 12A by brazing, by welding, or by application of a conductive adhesive.
Following the bonding process, the component 14A can be surface mounted to a supporting substrate 24A, such as a printed circuit board (PCB), to form an electronic assembly 22A. For attaching the component 12A to the substrate 24A, a second eutectic solder fillet 26A bonds the contact ball 10A to an contact pad 28A on the supporting substrate 24A. A solder reflow process, as previously described, can be used to form the eutectic solder fillet 26A, and to bond the contact ball 10A to the contact pad 28A.
One factor that can adversely affect the reliability of the assembly 22A during operation in different customer environments is fatigue failure of the contact ball 10A, particularly at the interface of the contact ball 10A with the bonding pad 12A. Typically, fatigue failures are induced by thermal expansion mismatches between the component 14A and the supporting substrate 24A. For example, if the component 14A comprises a first material, such as ceramic having a first CTE, and the supporting substrate 24A comprises a second material, such as FR-4 having a second CTE, cyclic loads can be placed on the contact ball 10A as the assembly 22A is thermally cycled during normal operation.
The forces acting on the contact ball 10A include tensile forces 30, moment forces 32, 34 and shear forces 36. If these forces are large enough, the contact ball 10A can separate from the bonding pad 12A on the component 14A. This separation can form an electrical open in the electrical path between the contact ball 10A and the bonding pad 12A on the component 14A. This separation also compromises the physical bond between the component 14A and the supporting substrate 24A. This problem is compounded because the area of interface between the contact ball 10A and the bonding pad 12A is relatively small. The forces are thus concentrated over a relatively small area.
FIGS. 1B-1F illustrate other types of components in which separation can occur between an external contact and a bonding pad on the component. In FIG. 1B, a component 14B includes a bonding pad 12B and a contact bump 10B formed on the bonding pad 12B. In addition, the contact bump 10B is bonded directly to an contact pad 28B on a supporting substrate 24B. In this example, the contact bump 10B can be formed on the bonding pad 12B using a deposition process, such as evaporation of a ball limiting metallurgy (BLM) and solder material through openings in a metal mask. In addition to the contact bump 10B, the ball limiting metallurgy (BLM) can include a multi layered stack (not shown) such as an adherence layer (e.g., Cr), a solderable layer (e.g., Cu) and a flash layer (e.g., Au). This process is also known as C4 technology, and is typically used to deposit contact bumps 10B directly onto aluminum bond pads on a semiconductor wafer or die. Alternately, other deposition processes, such as electroless deposition, or electrolytic deposition can be used to form the contact bump 10B. The contact bumps 10B can also comprise a pre-formed eutectic ball, which is placed on the contact pad 28B and reflowed, substantially as previously described for the non-eutectic contact ball 10A. In this case flux can be employed or reflow can be performed in an inert atmosphere.
In FIG. 1C, a component 14C includes a bonding pad 12C and a solder contact column 10C bonded to the bonding pad 12C using a eutectic solder fillet 16C. This type of component 14C is sometimes referred to as a ceramic column grid array (CCGA). The contact column 10C comprises an elongated member configured for bonding to an contact pad 28C on a supporting substrate 24C using a eutectic solder fillet 26C.
In FIG. 1D, a component 14D includes a TAB contact bump 10D bonded to a multi layered tape 38, that is similar to TAB tape. This type of component 14D is sometimes referred to as a TAB ball grid array (TBGA). For surface mounting the component 14D, the TAB contact bump 10D is configured for bonding to an contact pad 28D on a supporting substrate 24D using a eutectic solder fillet 26D.
In FIG. 1E, a component 14E includes a solder mask 40 having an opening 42 in which a solder mask contact ball 10E is formed. The opening 42 in the solder mask 40 facilitates alignment and bonding of the contact ball 10E to a bonding pad 12E on the component 14E. In addition in the completed assembly, the solder mask 40 insulates the contact ball 10E from adjacent contact balls 10E and other electrical elements on the component 14E, such as conductive traces. For surface mounting the component 14E, the contact ball 10E is configured for bonding to an contact pad 28E on a supporting substrate 24E using a solder fillet 26E.
In FIG. 1F, a component 14F includes a polymer tape 44 having a double sided stud contact bump 10F which comprises plated studs and a metal filled via in the polymer tape 44. The stud contact bump 10F is bonded to a bonding pad 12F on the component 14F using a eutectic solder fillet 16F. In addition, the stud contact bump 10F is bonded to a contact pad 28F on a supporting substrate 24F.
The present invention is directed to an improved semiconductor package in which external contacts on the component are rigidified by a separate polymer support member.